Volcanologists divide volcanoes into three main
groups, each of which is based on the shape of the
volcanoes and the type of material they are built of.

Shield Volcanoes are formed when a
large amount of free-flowing lava spills from a vent and
spreads widely. The lava gradually builds up a low,
broad,dome-shaped mountain around the vent. All the
Hawaiian Islands are composed of shield volcanoes that
have built above the sea surface from the ocean bottom.
The more lava making up a volcano, the greater will be
its slopes and the broader its base. If the lava erupts
from very long cracks in the earth, like it does in
Iceland, it spreads out in all directions to form
extensive lava plains that can reach heights of 5,000
feet or more.

If a volcano is built of nothing but solid fragments,
it is called a Cinder Cone. Such a
volcano is formed when large amounts of exploding gas
break up the lava into a bubbling froth. The froth is
ejected as a fine spray which solidifies into solid
particles, with little or no lava flowing as a liquid.
These volcanoes have much smaller bases than shield
volcanoes, but often reach much greater heights. In
addition, shield volcanoes typically have gradual slopes,
while cinder cones have steep, often almost vertical,
slopes. Parícutin in Mexico
is one of the most famous cinder cone volcanoes.

Most volcanoes are neither purely cinver cones nor
purely shields. Sometimes they erupt quiety, sometimes
explosively, and build a Composite Cone
of fragmental material interbedded with lava. Most of the
world's best-known volcanoes are compsoite types,
including Japan's Mount Fuji and Italy's Mount Vesuvius.
The eruptions that built them began with explosions of
solid material from a central crater or vent. Later on,
lava flowed through fissures in the sides of the
mountain, or out of secondary vents near the summit. As
such eruptions continue, the cones grow larger, many
reaching many thousands of feet in height.